During the past five years major innovations have brought SRI's ultrasound transmission imaging technology close to clinical application. The research proposed here comprises the next major step in the understanding and development of orthographic transmission imaging and will significantly enhance the technology's usefulness for medical diagnosis. Recent studies with our present imaging system indicate that significant diagnostic applications are likely, especially in orthopedics, pediatrics, and breast cancer screening. However, more basic understanding of the process of image formation is needed for optimization of each specific application. The major aims will be to: (a) analyze, model and experimentally verify interactions of ultrasound with inhomogeneous tissue; (b) develop on-line and off-line image processing to correct for degradation due to tissue inhomogeneities; and (c) evaluate the performance of the improved transmission-imaging camera. To fully utilize the benefits of this research, the laboratory version of camera will be retrofitted with various improved electronic, mechanical,and acoustic features already developed. These steps will provide a sound basis for estimation of camera performance in various clinical applications. Methodologies: (a) Scattering, reflection, absorption, and refraction of imaging wavefields will be treated independently, using both analytical and stochastic methods, computer modeling and imaging, and nonimaging experimental tests of the resulting hypotheses. (b) Linear, three-dimensional spatial filtering will be used to restore images degraded by out-of-focus tissue structures while statistical techniques will be used to enhance image features of diagnostic value. (c) The improvements in image quality resulting from these studies will be demonstrated in real-time operation of the camera. We expect to achieve significant enhancement of diagnostic capability. The long-term objectives are to investigate and demonstrate methods for increasing the range of clinical applications of orthographic ultrasonic imaging. We anticipate a clinical phase following the research and development now proposed; subsequent animal and human trials will demonstrate the newly acquired diagnostic capabilities. The proposed studies not only will lead to significant improvements in a unique form of noninvasive diagnosis but also will improve our understanding of the interactions of ultrasound and tissue--an important goal in its own right.